In engines used in automobiles, two-wheeled motor vehicles, and large generators, a starting operation is generally performed by a starter motor mounted on an engine. FIG. 7 is a perspective view showing an entire configuration of a starter motor 151 described above. The starter motor 151 is mounted on an engine (not shown) at an engine mounting surface 166 on a front side in the figure. The starter motor 151 is provided with an idle gear (middle gear) 152 that is movable in an axial direction and provided in a manner capable of meshing with and being separated from a ring gear (not shown) of the engine. The idle gear 152 is supported by an idle shaft 153 in a manner freely rotatable and movable in the axial direction, and meshes with a pinion (not shown) that is rotationally driven by a motor 154. The pinion is connected to a rotational shaft of the motor 154 via an overrunning clutch.
The idle gear 152 meshes with the ring gear when the engine is started. Along with turning ON of an ignition key switch, the idle gear 152 moves in the axial direction (an upward direction in FIG. 7) from a rest position to mesh with the ring gear, and rotates a crankshaft of the engine. If the engine is started and the number of revolutions of the pinion becomes higher than the number of revolutions of the motor 154, the overrunning clutch achieves an overrunning state, and the idle gear 152 and the pinion are in an idle running state. After the engine is started, the idle gear 152 moves in the axial direction, and is separated from the ring gear. In this manner, meshing between both of the gears is released, and the idle gear 152 returns to the rest position.
On the other hand, in the starter motor 151 described above, the idle shaft 153 is supported by bearing sections 156 and 157 formed on a gear cover 155, and is fixed on the gear cover 155 by an idle shaft fixing screw 158 (hereinafter abbreviated to the fixing screw 158) as shown in FIG. 7. In addition, to a top end section of the idle shaft 153, a dustproof cap 159 made of rubber is attached. FIG. 8 is an explanatory view showing a configuration of a screw fixing section of the idle shaft 153. As shown in FIG. 8, a female screw section 161 and a screw mounting hole 162 are formed on the same axis line on the bearing section 157 of the gear cover 155. A screw insertion hole 163 is formed on the idle shaft 153 corresponding to the female screw section 161 and the screw mounting hole 162. By screwing the fixing screw 158 in the female screw section 161 via the screw insertion hole 163 from the screw mounting hole 162, the idle shaft 153 is fixed on the gear cover 155.
FIGS. 9 and 10 are explanatory views showing processes of assembling and fixing the idle shaft 153 to the gear cover 155. FIG. 11 is a process chart showing a main assembling process in the above case. In the starter motor 151, an idle gear assembly 165 including the idle gear 152 is arranged on an idle gear mounting section 164 of the gear cover 155. In this state, the idle shaft 153 is inserted through an axis hole formed on the bearing section 156 from an upper end side of the gear cover 155 in the figures. At this time, the idle shaft 153 is inserted in the gear cover 155 in a manner such that the screw mounting hole 162 of the gear cover 155 and the screw insertion hole 163 of the idle shaft 153 match with each other to a certain extent (step (1)).
Next, the idle shaft 153 is inserted through the idle gear assembly 165. Thereafter, the idle shaft 153 is inserted in an axis hole formed on the bearing section 157. Then, positions of the screw mounting hole 162 and the screw insertion hole 163 are checked. In case these positions do not match with each other, the idle shaft 153 is rotated so that the holes are matched with each other (step (2)). Thereafter, as shown in FIG. 9, the fixing screw 158 is inserted from the screw mounting hole 162, and then the fixing screw 158 is screwed in the female screw section 161 via the screw insertion hole 163 (step (3)). In this manner, after the idle shaft 153 is fixed on the gear cover 155, the dustproof cap 159 is mounted (step (4)).    Patent Document 1: Jap. Pat. Appln. Laid-Open Publication No. 2003-239834
However, in the starter motor 151 described above, work is carried out after positions of the screw mounting hole 162 and the screw insertion hole 163 are made to match with each other to a certain extent when the idle shaft 153 is inserted. Nevertheless, when the idle shaft 153 is assembled, both of the holes need to be matched with each other for fixing the shaft with a screw. Then, if the positions of the holes do not match with each other to some extent at the time of screwing, the positions of the holes need to be matched again in a state where the idle shaft 153 is inserted. That is, checking of the positions of the holes and alignment of the holes is necessary at the time of fixing the shaft. Therefore, there has been a problem in which workability is not excellent and also additional steps are necessary to thereby increase the manufacturing cost. In particular, in the starter motor 151 in FIG. 7, the screw insertion hole 163 is positioned at 30° from the engine mounting surface 166 in consideration of interference with other sections. Therefore, alignment is difficult in comparison with a case in which the holes are at positions, such as positions in the right angle, where a reference can be easily set, and improvement in this respect has been required.
In addition, the alignment of holes described above is carried out in a manner such that a P part in FIG. 10 is held to rotate the idle shaft 153 while looking through the screw mounting hole 162 in order to match the screw insertion hole 163 with the screw mounting hole 162. With respect to this point, after the idle shaft 153 is inserted, a width of the P part is narrow and the idle shaft 153 is difficult to rotate, and also a part between the idle gear 152 and the idle shaft 153 is a grease application section which is not preferably touched by a worker for quality reasons. Further, a hand of the worker is stained at the time of alignment, which is not preferable in view of a working environment. In addition, in case the insertion and the screwing of the idle shaft 153 are carried out in an automated line, a step of detecting whether the holes match with each other or not is difficult to be set, and has been an obstacle to automation of assembly of the starter motor 151.
An object of the present invention is to provide a starter motor enabling screwing of an idle shaft to be fixed on a gear cover in a manner such that holes can be aligned easily and accurately when an idle shaft is mounted and such alignment is not carried out again.